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Author Topic: Motor Question  (Read 503 times)

Offline Howard Rush

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Motor Question
« on: March 11, 2011, 02:09:43 AM »
Suppose your airplane is cruising along in level flight and your speed controller makes a big step voltage input to the motor.  What's the max torque the motor can momentarily produce?  If the max burst current of my battery is 132 amps at 18.5V, I figure I'm entitled to 233 N-cm at 10KRPM.   The maximum Drehmoment I see on the Plettenberg Web site for the 20-16 is about 69 N-cm. 
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Offline John Rist

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Re: Motor Question
« Reply #1 on: March 11, 2011, 08:01:46 AM »
At full throttle the speed controller applies all of the battery voltage (less losses do to battery, wireing, and speed controller resistive losses) to the motor.  However the current does not jump to the max that the battery can deliver to a dead short.  Current is limited by the resistance of the motor windings and the back EMF generated by the motor.  The back EMF increases as the RPMs go up.  That is why a motor doesn't run away to unlimited RPMs.  It is also why a motor, with a stalled rotor, will burn up.  No RPMs - NO back EMF - current is limited only by the resistance of the windings which is typically low.  That's when you get the 100+ amps.  This also why speed controllers have soft start.  Give the motor a chance to spin up and generate back EMF.
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Offline Jason Greer

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Re: Motor Question
« Reply #2 on: March 11, 2011, 12:08:40 PM »
I dont think your battery is going to maintain 18.5 volts under a load of 132 amps.  The voltage drop under load would reduce the total power available.  I would think the voltage drop would be at least a couple volts at that current level.  I'm not smart enough to calculate torque numbers, but maybe this would explain why 233 N-cm would not be possible.  Wonder how long the 20-16 would last if short bursts of 2000+ watts were asked from it?
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Offline Dean Pappas

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Re: Motor Question
« Reply #3 on: March 11, 2011, 01:38:21 PM »
Hi Howard,
By the way, the ESC will not apply a stepwise change to the motor drive duty-cycle.
It's been taught not to ... LL~
It will ramp it, at a  rate that you have some flexibility in setting (throttle response spedd or something with a name like that).
This is done so that the resulting RPM ramp-up can always be tracked by the commutation phase-locked-loop.

later, Compadre,
  Dean
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Offline Tim Wescott

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Re: Motor Question
« Reply #4 on: March 11, 2011, 10:46:17 PM »
Jeze, Howard, what are you trying to do?  If you don't break the speed controller, or the batteries, or the motor, then yes, maybe you'll generate that much torque for a little while.

As pointed out, you'll probably really confuse the PLL in the speed controller, though.
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Offline Igor Burger

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Re: Motor Question
« Reply #5 on: March 13, 2011, 07:28:19 PM »
The moment is linear to the current in the winding and kV, so theoretical torque is:

torque = constant * current / Kv

where the constant depends on units, and if you use torque in Ncm , current in Amperes and Kv in rpm/V then it is ~1000 (depends little bit on loses)

the maximal current which you can deliver to the running motor from your battery is:

I = ( Ubat - actual_RPM / Kv ) / ( Rimot + Ribat )  .... I hope I did the math well ;D

BTW does not matter if you do that "step" in the power or if you just load the motor by large prop, the torque is the same  ;D
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